Correlation Analysis - Visual and numerical correlation between two sets of data with model, shapes or functions (FEA-Test, FEA-FEA, Test-Test).

Pretest Analysis

If a baseline finite element model is available, then this model can be used to simulate tests. This provides test engineers with optimal locations and directions to excite the structure, and to position measurement transducers. The FE model can be reduced and converted into a test model.

Selection of Candidate Sensor Locations - Use criteria like accessibility, cost, geometry (surface, edge or corner nodes) or any other user-defined criteria to select candidate locations. A set of fixed sensors can be added.

Sensor Placement Metrics - These are semi-automatic methods to find optimal exciter, suspension and measurement locations and directions. They are based on the observability of target modes using information on modal displacement or energy (kinetic or strain). Methods include: Normalized Modal Displacements, Nodal Kinetic Energy.

Sensor Elimination Methods - These methods iteratively eliminate sensors from the set of candidates in a way to optimally maintain linear independence or orthogonality between mode shapes. Methods include: Effective Independence Method, Elimination by MAC, Iterative Guyan reduction.

Mass Loading Evaluation - This tool evaluates the effect of accelerometer mass on the modal parameters.

Creation and Export of Test Model - Truncation of the FE model, conversion to test model and export to a modal test software. Automatic generation of tracelines between retained sensor locations. Directions normal to the surface can be obtained from the FE model. The measurement directions can be expressed as Euler angles.

Applications

Questions that can be answered with pretest analysis include:

How many modes can be expected in a given frequency range?

What are the optimal location and directions for sensors, exciters and suspensions from a set of candidate locations?

Create a test model from a reduced finite element model and export in a format readable by modal test packages.

Determine the directions normal to the surface of curved surfaces from the finite element model and use this information for decomposing modal test displacements in Cartesian coordinates.

What are the measurement directions expressed as Euler angles?

Assess the influence of the accelerometer mass on the modal parameters.

Using the pretest analysis tools it is possible to plan an optimal modal test strategy early in the project and increase quality of modal data for validation and updating of FE models.

Key Features

Plan a test strategy early in the project.

Easily find optimal location for sensors, exciters and suspension.

Fast creation of a test model from a baseline FE model.

Increase quality of modal test data for validation and updating of FE models.

Correlation Analysis

Correlation analysis quantitatively and qualitatively compares 2 sets of analysis results data. Usually this is a FEA and a test database that are imported in the FEMtools database. However, the tools can be used for FEA-to-FEA and test-to-test correlation as well.

Overview

Spatial correlation - Compares location in space between response locations resulting in a table with mapped degrees-of-freedom. This may require changing orientation and scaling of the models, which can be done in a manual way or using automatic tools.

Global shape correlation - Globally compares shapes using various criteria. The result is used for shape pairing.

Local shape correlation - analyzes local spatial correlation between shapes. Results can be interpreted to localize modeling deficiencies and serve as guideline for selecting model updating parameters.

FRF correlation - Analyzes correlation between FRF functions, either globally between 2 functions or shape and amplitude correlation functions for a set of FRF pairs as function of frequency.

Correlation coefficients - Calculates values of error functions from a selection of reference responses. These functions are used in model updating to monitor the distance between the updated model and a reference.

Results from correlation analysis are used to define targets for FE model updating. Similar mode shapes can be identified in the FE and test database thus providing residues in terms of resonance frequency differences, MAC, modal displacements.

Another application is to provide the analyst with information that can only be measured. An example is modal damping, used in modal superposition methods. Modal damping can be obtained experimentally and applied to the analytical mode shape that, using correlation analysis, was found to best match the experimental one.

Modal correlation analysis is also used to scale test mode shapes obtained by output-only modal analysis. The same scaling as used by the analytical mode shapes (e.g. unity modal mass), can be applied to the correlated test modes.

Unlike global correlation analysis, spatial correlation methods are used to identify areas of better or poorer correlation, which when linked to structural information, can be interpreted in terms of 'modeling error'. Depending on how these tools are used, the results help with the selection of updating variables (parameters), or are used to assess structural damage.

Key Features

FEA-Test, FEA-FEA, Test-Test Correlation.

DOF pair table definition, ranking and filtering.

Automated or manual model mapping.

Directional and multi-step pairing.

Efficient processing of large data sets (point clouds).

Static, modal and operational shape correlation analysis using MAC.

Mode shape auto- and cross-orthogonality check using full or reduced system matrices.

Automatic mode shape pairing using MAC or ortogonality matrix.

Automated support for mode shape pairing in case of double modes (axisymmetric structures).